A lot of people (especially working people with mobile devices like notebook/netbooks) need to use different proxy servers at home and office. There are several Firefox extensions available to achieve the required functionality but IMHO Multiproxy Switch(Mozilla Addon Page) is the best because

Its simple and easy to use. It does what it should. No fancy/extra terrestrial stuff. Just switch proxies

Easy and Firefox like interface to specify different proxies. Many extensions add their own fancy interfaces for specifying proxies which eventually suck big time.

I am a fan of this one. The No-Proxy list. I could never understand those regular expression based no-proxy lists in FoxyProxy. Multiproxy Switch has Firefox like No-Proxy list which rocks and understandable

If you happen to come across a better proxy switcher for Firefox, do let us know